1. Field of the Invention
The present invention relates to a fluid bearing device, particularly, relates to a fluid bearing device, which is suitably mounted to a disc driving motor to be installed in a hard disc drive.
2. Description of the Related Art
The Japanese publication of unexamined patent applications Nos. 2002-101610 and 8-210364/1996 disclose a motor mounted with a conventional fluid bearing device respectively. Such a motor mounted with a conventional fluid bearing device is exemplified in FIG. 6.
FIG. 6 is a cross sectional view of a motor mounted with a conventional fluid bearing device according to the prior art.
In FIG. 6, a motor 500 is a disc driving motor utilized for a hard disc drive, and the motor 500 is composed of a stator 500S and a rotor 500R.
The stator 500S is constituted by a motor base J13, a sleeve J9, which is made from brass and fixed on an inner circumferential surface of a raised section J13a in an annular shape that is formed on a center portion of the motor base J13, and a stator core J14, which is fixed on an outer circumferential surface of the raised section J13a. 
The rotor 500R is constituted by a hub J2 and a ring magnet J6, which is fixed to the hub J2.
A hard disc not shown is mounted on an outer circumferential surface J2a of the hub J2.
Further, a shaft J1 made from stainless steel is fixed into a center hole J2b of the hub J2.
In the above-mentioned configuration, the sleeve J9 supports the shaft J1 in a thrust direction and a radial direction through a fluid bearing. Consequently, by the fluid bearing, the rotor 500R is supported rotatable freely with respect to the stator 500S.
A fluid bearing in the radial direction is constituted by an outer circumferential surface J1a of the shaft J1, an inner circumferential surface J9a of the sleeve J9 and lubrication fluid 20 that is filled in a gap between the outer circumferential surface J1a and the inner circumferential surface J9a. The lubrication fluid 20 also constitutes a not shown fluid bearing in the thrust direction.
A taper seal section JTS, which seals in the lubrication fluid 20, is provided on an end portion of the sleeve J9 on the side confronting with the hub J2. The taper seal section JTS is formed by the inner circumferential surface J9a of the sleeve J9 and the outer circumferential surface J1a of the shaft J1, which confronts with the inner circumferential surface J9a. 
Further, a filling amount of the lubrication fluid 20 is regulated such that a fluid level 20Ja of the lubrication fluid 20 remains within the taper seal section JTS.
In the motor 500 mounted with the above-mentioned conventional fluid bearing, the lubrication fluid 20 is filled inside the bearing. By using pressure of the lubrication fluid 20 generated by revolution of the rotor 500R, the shaft J1 is supported rotatable freely without contacting with other members such as the sleeve J9.
Further, the taper seal section JTS for sealing in the lubrication fluid 20 is provided on an open-end section of the fluid bearing. The taper seal section JTS is formed in a shape, which prevents the lubrication fluid 20 from leaking out even though the lubrication fluid 20 is expanded by temperature rise caused by revolution of the rotor 500R.
In other words, the taper seal section JTS is formed in a shape that enables to reserve a prescribed capacity of the lubrication fluid 20.
By using some examples, the expansion of the lubrication fluid 20 is described next.
A coefficient of linear expansion of the shaft J1, which is made from stainless steel, is 10.5×10−6/° C. On the contrary, a coefficient of linear expansion of the sleeve J9, which is made from brass, is 17×10−6/° C.
Further, a usable upper limit temperature of the motor 500 is generally 80° C. Consequently, an increment of temperature is 55° C. when an ambient temperature rises from the room temperature of 25° C. to the upper limit temperature of 80° C.
The fluid bearing is constituted at the room temperature of 25° C. such that the shaft J1 made from stainless steel of which an outer diameter is 4.0000 mm and a length is 20.000 mm, is inserted into a center hole of the sleeve J9 made from brass, wherein an inner diameter of the center hole is 4.0050 mm and a depth or a length of the center hole is 21.000 mm.
Further a capacity or a cubic volume for reserving the lubrication fluid 20 of the gap between the sleeve J9 and the shaft J1 except the taper seal section JTS is 13.227 mm3.
When a temperature rises from the room temperature to 80° C., the inner diameter of the center hole of the sleeve J9 changes into 4.0087 mm, the outer diameter of the shaft J1 changes into 4.0023 mm, and the cubic volume of the gap changes into 13.227 mm3. In other words, the cubic volume increases 1.0154 times and a rate of the increase is 1.54%.
On the other hand, the taper seal section JTS is formed such that a minimal diameter is 4.0050 mm, a maximal diameter is 4.2050 mm, and a length in the axial direction is 2.1000 mm at the room temperature of 25° C. respectively. A cubic volume of the taper seal section JTS is 1.4091 mm3 at the room temperature of 25° C.
Accordingly, when a temperature rises from the room temperature to 80° C., the cubic volume of the taper seal section JTS changes into 1.4306 mm3. In other words, the cubic volume of the taper seal section JTS increases 1.0153 times and a rate of the increase is 1.53% at the temperature of 80° C.
On the contrary, a coefficient of linear expansion of oil, which is commonly used as the lubrication fluid 20, is 8×10−6/° C. When temperature rises from the room temperature of 25° C. to 80° C., an increment of temperature increase is 55° C. Consequently, a rate of increase of cubic volume of the oil is 4.40%.
As mentioned above, with respect to increase of a capacity or a cubic volume caused by temperature rise, an amount of increase of a cubic volume of lubrication fluid itself is larger than a total amount of capacity increase of the cubic volume of the gap between the sleeve J9 and the shaft J1 except the taper seal section JTS for reserving the lubrication fluid 20 and the capacity increase of the taper seal section JTS.
Accordingly, the fluid level 20Ja of the lubrication fluid 20 rises in the taper seal section JTS in response to temperature rise.
More specifically, in the case a distance between an end surface J9b of the sleeve J9 and the fluid level 20Ja in FIG. 6 is assumed to be 1.100 mm at the room temperature of 25° C., subtracting total amount of increased capacity for reserving the lubrication fluid 20 from volume expansion of the lubrication fluid 20 itself becomes 0.3824 mm3 when temperature rises up to 80° C. In this case, the fluid level 20Ja rises by 0.479 mm. Consequently, a length of the taper seal section JTS in the axial direction is designated such that the lubrication fluid 20 does not overflow even when the fluid level 20Ja rises as mentioned above.
In the case of actually designing a motor mounted with a fluid bearing, a shape of a taper seal section must be designed in consideration of dimensional variations possibly occur while mass producing, a decreased amount of lubrication fluid due to evaporation, and a waving fluid level of the lubrication fluid caused by shock applied externally, in addition to the rise of the fluid level.
However, a total thickness of a motor is strictly restricted. For example, in the case of installing the motor in a hard disc drive, the total thickness of the motor is regulated to be 7.5 mm or less. If a length of a taper seal section in the axial direction is made longer, a bearing span in the thrust direction is made shorter in response to the extended length of the taper seal section, and resulting in increasing burden on the bearing.
Accordingly, there existed a problem such that rotational vibration of a rotor increased.
With respect to the above-mentioned problem, it has been studied that a material of sleeve has been changed into another material having a coefficient of linear expansion, which was larger than that of other members.
According to the above-mentioned configuration such that a coefficient of linear expansion of the sleeve is larger than that of other members, an inner diameter of a sleeve expands in response to a rising temperature, and resulting in increasing a capacity of a taper seal section. Consequently, rise of a fluid level of lubrication fluid is suppressed. However, a section for reserving the lubrication fluid other than the taper seal section, that is, a gap between the shaft and the sleeve is extremely small, so that the gap drastically expands and stiffness of the bearing is deteriorated, and resulting in a further problem such that rotational vibration of the rotor extremely increases.